It is known to provide a lithium-ion secondary battery with a negative electrode formed by using a material other than metallic lithium and capable of absorbing and discharging lithium ions thereby restraining deposit of dendrite in comparison to a negative electrode made of metallic lithium. Such known battery helps to prevent the occurrence of a short circuit between positive and negative electrodes and thus improves safety. In addition, such batteries have reasonably good capacity and energy density.
Nevertheless, there is an ongoing demand for such type of Lithium-ion secondary batteries to have yet higher capacities, energy densities and long battery life. This is particularly apparent in the automotive and portable electronic devices. Such batteries must sustain repeated charging and discharging at a high electric current for up to tens of thousands of cycles without noticeable loss of capacity.
In general, higher capacity is obtained by decreasing the electric resistance within the battery.
It has already been suggested to comply with these demands by: (a) having a positive-electrode material made of a lithium metal oxide and a negative electrode material made of carbon (see patent documents 2, 3 and 4), (b) increasing the specific surface areas of particles of a reactive substances of the battery by decreasing the diameters of the particles or increasing the surface area of the electrode, (c) by decreasing liquid diffusion resistance by thinning the separator membranes.
Of course, when the diameter of the particles of reactive material of the battery are made smaller, the specific surface areas of the particles increase. This in turn necessitates the amount of a binder to be increased. As a result, it is difficult to provide a high capacity battery when more binder is present. In addition, the positive-electrode and negative-electrode materials may peel or drop from a metal foil used as an electricity collector, which may result in the occurrence of an internal short circuit inside the battery or some decrease in the output voltage of the battery and thermal runaway. Thus, the capacity and safety of the lithium secondary battery are impaired.
To increase the adherence of the metal foil to the positive-electrode and negative-electrode materials, methods of altering the binder substance are known (patent document 1).
Nevertheless, when the battery is cyclically charged and discharged at a high electric current, the positive-electrode and negative-electrode materials expand and contract. Thus conductive paths of particles between the positive and negative electrodes are impaired. As a result, soon after the initial charging and discharging cycles, the battery loses capacity and has a short life.
Recently, a lithium-containing metal phosphate compound such as an olivine-type lithium iron phosphate has attracted rising attention as active substance of the positive electrode for the lithium-ion secondary battery (patent documents 5, 6). Although cheap to purchase, the active substance suffers from high electric resistance and thus reduced capacity.